Diagnostic ultrasound imaging systems provide a comprehensive evaluation of a subject's health condition, The efficacy of ultrasound techniques have resulted in the widespread acceptance of ultrasound imaging by both patients and physicians. In general, diagnostic ultrasound imaging systems generate images of anatomical structures within the patient by transmitting ultrahigh frequency soundwaves (typically in the order of 3.0-10.0 MHz) and then analyzing the waves reflected from the body structure. The most widely used ultrasonic diagnostic systems display structural information of organs in the form of two-dimensional images of selected cross sections of the organ. Typically, the ultrasound is swept across the organ in the form of a "cross sectional scan". The scan is ordinarily performed in real time so that the dynamics of anatomical structures can be visualized.
In presently available ultrasound systems, in addition to anatomical information, blood flow information is often provided by utilizing the Doppler principle, or other known techniques. A beam, comprising pulses of ultrasonic energy is directed toward a blood vessel in which blood flow information is desired. For example, a scan is conducted on the placenta wherein the blood vessels between the mother and the embryo are interfaced, but not joined. To use the Doppler principle, the beam of ultrasonic energy is directed toward a blood vessel. Moving blood cells reflect the ultrasound energy and either increase or decrease the frequency of the reflected energy depending on the direction of the blood flow in accordance with the well known Doppler principle.
The magnitude of the frequency shift and the direction of the shift are detected so that the velocity and the direction of the blood flow may be ascertained. Such Doppler ultrasound apparatus also typically provides the usual anatomical information using conventional diagnostic ultrasound techniques.
One form of examination is to thoroughly interrogate blood flow at a certain point, typically 1-2 mm in size as a function of time for periods of time that typically are several heart cycles in length. The certain points interrogated are often referred to as "gates" or "sample volumes". The information obtained this way is mainly the flow velocity, volume flow and velocimetry indices: such as PI (Pulsitility Index), RI (Resistance Index), etc. Another form of examination is to interrogate a multiplicity of gates during the same time period. Thus, either one gate is given an extensive interrogation or a plurality of gates are cursorily interrogated during the interrogation period.
The ultrasound equipment now in use, however, fails to provide for mapping of flow indicies which describe the nature of the flow other than average or maximum velocity of the flow. Thus, while the analysis of blood flow using ultrasound has found a variety of applications in recent years, it has not been used to perform effective mapping of flow parameters other than average or maximum velocity.
One example of the application of flow indicies, is to study blood perfusion in the placenta. The studies of this type are used to monitor fetal growth based on the fact that normal fetal growth depends on an adequate supply of oxygen and nutrients which are generally carried to the fetus by the fetal blood through the umbilical placental circulation throughout pregnancy. The studies of the umbilical placental circulation of the human fetus have been greatly facilitated by the use of the aforementioned ultrasound analysis of the flow. Recently, there have been some studies wherein the waveforms of the flow velocity have been studied. However, most of these studies have focused on the umbilical artery. The characteristics of the fetal circulation further downstream to the umbilical artery have seldom been studied due to practical reasons and no effective method of using clinical ultrasound has been developed until now to augment such studies.
Another example where studies of flow parameter characteristics would be beneficial is the examination of whether or not a suspicious mass might be a malignant tumor. It is known that malignant tumorous masses are generally accompanied by anglogenesis which results in increased diastolic blood flow. This blood flow can be measured and characterized with spectral flow analysis. For example, in the past, procedures using spectral Doppler for characterizing tumors have not proven fully successful because the required thorough investigation of the tumor area was not practical with present state of the art. Presently the blood vessels are detected using color flow imaging. Each point is then analyzed separately using spectral analysis. This is a very tedious and time consuming procedure, and is therefore limited to a relatively small number of points, which may sometimes be insufficient for a reliable diagnosis.